The concrete towers for wind turbines can be divided into sections, which are hollow segments that can in turn be divided into sectors or shell segments, to allow the sectors or shell segments to be transported by road on wheeled vehicles. The sections are pre-assembled near the final tower location by joining the shell segments that make up the section in a phase called pre-assembly. The pre-assembled sections are then hoisted by a crane and stacked on top of each other to form the tower.
Pre-assembly is normally done on assembly bases, on which the shell segments are arranged vertically until the section has been completed. Pre-assembly of sections, among other disadvantages, requires a horizontal ground on which to carry out the process, also, due to the size of the shell segments, wind acts on the surface, which complicates assembly and subjects the pre-assembly base to high compression and/or tensile stresses. Another disadvantage is that the shell segments can begin to resonate with the frequency of the wind.
To support the loads of the tower and nacelle, as well as the wind load, a foundation must be constructed under the tower.
Current foundation design involves structures that are joined at the top to the bottom section of the wind turbine tower, with the lower part supported on or
buried in the ground. The function of the foundation is to transmit the loads to the ground, guaranteeing stability of the tower and distributing the load on a broader ground element.
There are foundations with constant and variable thicknesses. Examples of foundations with constant thickness are those with a square or oval horizontal layout and a cross-section that remains constant throughout the element. However, foundations with variable geometry or thickness, for example with a truncated cone shape, offer the advantage of reducing the amount of material required for construction. The base with the larger diameter is normally on the bottom, and the base with the smaller diameter on the top. The disadvantage of this foundation arrangement is that the slanted side surfaces that extend between the upper and lower bases are not flat, because the side area has slanted walls. This prevents this section of the foundation from being used as a pre-assembly base for the shell segments because, as mentioned before, a horizontal surface is required.
In order to resolve this problem, the foundation can be arranged upside down, in other words, with the base with the smaller diameter on the bottom and the base with the larger diameter on the top and in contact with the bottom section of the tower, so that this surface can be used as the horizontal surface on which to pre-assemble the sections. The main disadvantage of this solution is that it requires a larger diameter footer, and also uses more steel and/or concrete to withstand the same stresses. This is because:
the behavior of the ground is worse due to the footer geometry, because the ground has less rotational resistance;
since there is no earth on top of the footer, it does not have the extra weight of the ground to provide increased stability to the tower base.